1. Field of the Invention
The present invention relates to a semiconductor device having a TFT (thin film transistor) provided on an insulating substrate such as glass, and a method of manufacturing the semiconductor device.
2. Discussion of the Related Art
As the semiconductor having the TFT formed on the insulating substrate made of glass or the like, there have been known an active liquid crystal display device, an image sensor and the like, which use the TFT for driving a pixel.
A thin film silicon semiconductor is generally used for the TFT used in these devices. The thin film silicon semiconductor is roughly classified into the amorphous silicon semiconductor (a-Si) type and the crystalline silicon semiconductor type. The amorphous silicon semiconductor is most generally used because the manufacturing temperature is low, it can be relatively readily manufactured by a vapor phase method, and the mass productivity is sufficient. However, since the physical properties of the amorphous silicon semiconductor is inferior to the crystalline silicon semiconductor such as the electrical conductivity or the like, there is a strong demand to establish a method for manufacturing the TFT formed of the crystalline silicon semiconductor for the purpose of obtaining the higher speed characteristics in the future. As the crystalline silicon semiconductor, there have been known non-single crystalline silicon semiconductors such as polycrystalline silicon, microcrystalline silicon, amorphous silicon containing crystal components, semi-amorphous silicon having an intermediate state between the crystal property and the amorphous property, and the like. Hereinafter, the non-single crystalline silicon semiconductors having these crystal properties are called a crystalline silicon.
As a method of obtaining the thin film silicon semiconductors with these crystal properties, there have been known the following methods.
(1) A crystalline film is directly formed at the time of film formation.
(2) The energy of a laser illumination is applied to an amorphous semiconductor film which has been previously formed to provide the crystal property.
(3) A heat energy is applied to an amorphous semiconductor film which has been previously formed to provide the crystal property.
However, in the method (1), it is technically difficult to uniformly form a film having the excellent semiconductor physical properties all over the upper surface of a substrate. Further, since the film forming temperature is high, that is, 600xc2x0 C. or more, there rises such a problem in costs that an inexpensive glass substrate cannot be used. In the method (2), in the case of an example of an excimer laser which is most generally used now, there rises a problem that a through-put is low because a laser beam radiated area is small. Furthermore, the stability of the laser beam is insufficient to uniformly treat the entire upper surface of a large-area substrate, whereby it strongly seems as if this method is the technique for the coming generation. In the method (3), there is advantageous in that this method can cope with the large-area of the substrate in comparison with the methods (1) and (2). However, a high temperature of 600xc2x0 C. or more is required as a heating temperature, and taking the inexpensive glass substrate used into consideration, it is necessary to further decrease the heating temperature. In particular, the current liquid-crystal display unit advances to a large screen, and for that reason, it is necessary to use a large-scale glass substrate likewise. When such a large-scale glass substrate is used, there rises a serious problem that the contraction or strain of the substrate in the heating process essential to the semiconductor manufacture makes the accuracy in mask matching or the like deteriorate. In particular, in the case of the 7059 glass which is most generally used now, the temperature of the strain point is 593xc2x0 C., whereby the conventional heat crystallization method causes the substrate to be largely deformed. Moreover, in addition to the temperature problem, since the current process requires heating time of several tens hour or more necessary for crystallization, it is also necessary to shorten the heating time.
The present invention has been made to eliminate the above-mentioned problems, and an object of the invention is to provide a process of lowering a temperature necessary for crystallization and reducing a time therefor in a method of manufacturing a thin film formed of a crystalline silicon semiconductor by application of a method of crystallizing a thin film formed of an amorphous silicon by heating. The crystalline silicon semiconductor prepared by a process of the present invention has the physical properties not lower than those prepared by the conventional process, and applicable even to the active layer region of a TFT.
The inventors have conducted the following experiments in the above-mentioned method of forming an amorphous silicon semiconductor film by the CVD method or the sputtering method to crystallize the film thus formed by heating, and considered the experiment result.
First, the mechanism of forming the amorphous silicon film on a glass substrate to crystalize the film by heating has been investigated. As a result, it has been observed that the crystal growth started from an interface between the glass substrate and the amorphous silicon, then developed into the columnar shape perpendicular to the front surface of the substrate when it has the thickness of a certain degree.
It is considered that the above-mentioned phenomenon is caused by the fact that a crystalline nucleus forming a base of the crystal growth (the source forming a base of the crystal growth) exists in the interface between the glass substrate and the amorphous silicon film and the crystal grows from the nucleus. Such a crystalline nucleus is considered to be of a bit of impure metallic element which exists on the surface of the substrate or the crystalline component of the glass surface (it is considered that the crystalline component of silicon oxide exists on the surface of the glass substrate as called the crystallized glass).
Therefore, it was considered that the temperature of crystallization can be lowered by more positively introducing the crystalline nucleus, and for the purpose of confirming the effect, a bit of other metals was formed on the substrate, and a thin film of the amorphous silicon was then formed thereon. Thereafter, the experiment of crystallization by heating was conducted. As a result, it was confirmed that, in the case of forming several metals on the substrate, the temperature of crystallization was lowered, and it was expected that there occurred crystal growth which had the foreign matter as the crystalline nucleus. Therefore, the mechanism of a plurality of impurity metals which could lower the temperature has been investigated in more detail.
The crystallization can be classified into two stages, that is, an initial nucleus production and the crystal growth from the nucleus. The speed of the initial nucleus production was observed by measuring a time until fine crystals occurred in the dot pattern at a given temperature. That time was reduced in any cases of the thin films forming the above impurity metals, and the effect of lowering the temperature of crystallization when the crystalline nucleus was introduced was confirmed. Further, the growth of a crystal particle after nucleus production was investigated while changing the heating time. As a result, though this was beyond all expectations, it was observed that even the speed of crystal growth after the nucleus production was remarkably increased in the crystallization of the amorphous silicon thin film formed on the film of a certain metal. This is beyond all expectations. This mechanism will be described in more detail later.
In any case, it was ascertained that, in the case of forming a thin film made of amorphous silicon on a film containing a bit of metal of a certain kind and thereafter crystallizing it by heating with the above-mentioned two effects, the sufficient crystal properties, which could not be conventionally expected, could be obtained at a temperature of 580xc2x0 C. or less and for about four hours.
As examples of the impurity metals having such effects, there are indium, tin, antimony, germanium, thallium, lead, bismuth, and zinc. These impurity metals are close to silicon in group or period, and cooperate with silicon to readily produce a compound. Also they are commonly materials of a relatively low melting point, and hereinafter they are referred to as xe2x80x9clow melting point materialxe2x80x9d in the specification. Also, there are Lanthanide series elements having the effect of lowering a temperature as the result of an experiment except for those elements. They are used as a hydrogen occlusion alloy and commonly highly reactive to hydrogen. In this specification, they are referred to as xe2x80x9ccatalytic metalxe2x80x9d. Further, according to the acknowledge of the inventors, materials having the above-mentioned materiality among III, IV and V group elements can be used as the above-mentioned catalytic metal in principle. That is, there can be used B, Al, Ga, In, Tl, Sc, Y and Lanthanide of Group III elements, C, Ge, Sn, Pb, Ti, Zr, and Hf of group IV elements, and N, P, As, Sb, Bi, V, Nb and Ta of group V elements. Preferably, the above-mentioned indium (In), tin (Sn), antimony (Sb), germanium (Ge), thallium (Tl), lead (Pb), bismuth (Bi) and zinc (Zn) are useful to remarkably obtain the effect. Also, although zinc belongs to group II elements, it can be used as the above-mentioned low melting point metal because of a low melting point.
An example of how tin typical as the low melting point metal material provides the effect will be described. In the case where a thin film made of amorphous silicon formed by the plasma CVD method on a substrate (Corning 7059) which was not subjected to any processing, that is, on which a thin film made of a small amount of tin was not formed, was crystallized by heating in a nitrogen atmosphere, if a heating temperature was 600xc2x0 C., 10 hours or more were required as a heating time. However, in the case of using a thin film made of amorphous silicon formed on a substrate on which a thin film made of a small amount of tin has been formed, the same crystallizing state could be obtained by heating for about one hour. At this time, the judgement of crystallization was made by using the Raman spectroscopic spectrum. As is apparent only from this fact, the effect of tin is very large.
As is understood from the above description, in the case where a thin film made of amorphous silicon is formed on a thin film made of a small amount of low melting point metal or a small amount of catalytic metal on the substrate, it is possible to reduce a time and temperature required for crystallization. On the assumption that this process is used for manufacturing the TFT, description will be given in more details. Although it will be described later, even if the thin film of the low melting point metal was formed on the amorphous silicon, the same effect could be obtained instead of forming the amorphous silicon on the metal, and also it was the same as in the case of ion implantation.
Therefore, hereinafter, in this specification, a chain of these processes are referred to as xe2x80x9ca small amount of low melting point metal additionxe2x80x9d and xe2x80x9ca small amount of catalytic metal additionxe2x80x9d. These metals may be added when forming the thin film made of amorphous silicon.
First, a method of adding the low melting point metal will be described. It is identified that the addition of a small amount of a low melting point metal may be made by a method of forming a thin film made of the small amount of low melting point metal on a substrate and then forming a film made of amorphous silicon thereon, or by forming the film of amorphous silicon in advance and then forming the thin film of the small amount of low melting point metal thereon, because both the methods have the temperature lowering effect likewise, and the film forming methods can be made by the sputtering method or the vapor deposition method so as not to be limited to a specific method. However, when the thin film made of a small amount of low melting point metal film on a substrate, it is remarkable in effect to form the thin film of silicon oxide on the substrate and then to form the thin film of a small amount of low melting point metal rather than to form the thin film of a small amount of low melting point metal directly on a substrate made of the 7059 glass. The reason for the above can be considered that it is significant for the crystallization at a low temperature in this case to bring silicon in direct contact with the low melting point metal, and in the case of the 7059 glass, the components except for silicon impede the contact or reaction of both the silicon and the low melting point metal. The entirely same adding method can be also applied to the catalytic metal.
As the method of adding a small amount of material, even in the case of adding a small amount of material by ion implantation in place of forming the thin film in contact with the upper or lower portion of amorphous silicon, the substantially same effect was confirmed. As the amount of the low melting point metal, for example, when the amount of adding tin is 1xc3x971015 atoms/cm3 or more, the temperature lowering was confirmed. However, when the adding amount is 1xc3x971021 atoms/cm3 or more, the shape of a peak of the Raman spectroscopic spectrum is clearly different from that of silicon simple substance. Therefore, it appears that the actual useable adding amount ranges from 1xc3x971015 atoms/cm3 to 5xc3x971019 atoms/cm3. It is also necessary to restrain the adding amount to 1xc3x971015 atoms/cm3 to 1xc3x971019 atoms/cm3 in view of the fact that it is used for the active layer of the TFT as a semiconductor materiality.
Subsequently, the crystallization mechanism supposed when adding a small amount of low melting point metal will be first described.
As mentioned above, in the case of adding no catalytic metal for low temperature crystallization, nucleuses occur from crystalline nucleuses such as the interface of a substrate or the like at random, and crystals grow from the nucleuses likewise at random. It was reported that there was obtained a crystal relatively oriented at (11 0) or (111) depending on the manufacturing method, and it was observed that the crystal grows substantially uniformly over the entire thin film.
For the purpose of confirming this mechanism, analysis was made by a DSC (differential scanning calorimeter). An amorphous silicon thin film formed on a substrate by the plasma CVD method was located in a sample vessel in such a condition where it is mounted on the substrate, and then a temperature in the vessel went up at a given speed. As a result, a definite heating peak was observed at around 700xc2x0 C., and crystallization was also observed. This temperature is naturally shifted as the temperature rising speed is varied. For example, when the speed was 10xc2x0 C./min, crystallization started at the temperature of 700.9xc2x0 C. Subsequently, the measurements were made at three different temperature rising speeds, respectively, and then the activation energy of crystal growth after an initial nucleus was generated was obtained by the Ozawa method. Consequently, a value of approximately 3.04 eV was obtained. As the result that a reaction rate equation was obtained from fitting for a theoretical curve, it was proved that the crystallization mechanism is best explained by a disordered nucleus generation and its growth model, and there was confirmed the propriety of such a model that the nucleuses are produced from the crystalline nucleuses such as the interface of a substrate, etc. at random, and crystals grow from the nucleuses.
The entirely same measurement as in the above description was made to a case of adding the low melting point metal, in this example, of adding a small amount of tin. As a result, when a temperature went up at the speed of 10xc2x0 C./min, crystallization started at 625.5xc2x0 C., and the activation energy of the crystalline growth obtained from a chain of these measurements was 2.3 eV. Thus, it was numerically apparent that the crystalline growth was facilitated.
The reason why the crystallization starting temperature is made low is relatively readily considered as the result of the effect of foreign matters as described above. However, what is the cause of the temperature to be lowered up to the activation energy of the crystallization growth? As this cause, the inventors consider the following reasons.
The rate-determining process in crystallization of amorphous silicon is generally said to be a self-diffusion of silicon atoms. If it is a fact, then it is better to make the diffusion speed higher. However, in the case of crystallization from amorphous silicon, it should be considered as crystallization from an undiluted solution having a very high viscosity, which is different from crystallization of a crystal from aqueous solution or the like. Therefore, since a difference in density between a crystalline portion and its periphery is very small, atoms cannot be readily moved. In such circumstances, in order to give atoms mobility, there are considered the following three methods.
1. The viscosity of an amorphous film is changed to provide circumstances in which silicon atoms are liable to move.
2. A large amount of defects, depletions or the like are introduced to provide circumstances in which silicon atoms are liable to move.
3. The Coulomb""s force or the like is exerted to change the drive force of crystallization.
These three methods are not independent of each other, but it is considered that two or three of these methods are simultaneously satisfied depending upon the kind of material to be added.
The low melting point metal materials added in this case are considered to almost satisfy the above method 1. Also, as to III and V group materials, it is expected that positive or negative depletions or the like are formed in order to satisfy the principle of electrical neutrality, and therefore it is considered to satisfy the above method 2. Likewise, III and V group materials make the Fermi level shifted by generation of a level caused by these materials, and in the case where the amount of shift is different between an amorphous portion and a crystal portion (in general, it is considered that the amount of shift is differentiated by influence of the level of a middle gap in the amorphous portion), it is considered that the drive force caused by the difference in the amount of shift is generated, thereby enabling the temperature of crystallization to be lowered. As a result of supporting this mechanism, there is the fact that the temperature is difficult to be lowered in the case where III and V group materials are simultaneously added.
Subsequently, the crystallization mechanism in the case of adding a catalytic metal will be described.
Similarly in this case, as a result of measuring the activation energy of a crystal growth by a DSC measurement, it was identified that the energy was lowered to approximately 2.1 eV, and crystallization was facilitated. The following mechanisms are considered as this reason.
As mentioned above, these xe2x80x9ccatalytic metalsxe2x80x9d have a very high reactivity to hydrogen. Therefore, it is expected that the catalytic metals are coupled with hydrogen coupled with silicon in priority with the result that a large amount of dangling bonds are generated. The large amount of dangling bonds are considered to satisfy the above-mentioned method 2 for giving the atoms mobility. Also, it is considered that the generation of depletions or the like is caused by the difference in electro-negativity between silicon and Lanthanide to satisfy the principle of the electrical neutrality. Even though it is not such a case, it is required that Lanthanide is electrically greatly charged. In such a case, it is assumed that there is a possibility that the drive force is generated with the movement of the Fermi level.
Next, the crystal forms of crystalline silicon films obtained by adding a small amount of the above-mentioned low melting point metal or catalytic metal will be described. Since both of cases exhibited almost the same crystalline configuration, it seems that they are caused by the liability to movement of silicon atoms.
The metals added (both of the low melting point metals and the catalytic metals) diffuse to a considerably broad region at a temperature of the crystallization temperature or less. This fact was identified by a SIMS (secondary ion mass spectrometry). Then, as a result, even in the diffused region, the temperature of crystallization was lowered. Also, it was proved that the crystal form in the direct adding region is different from that in the peripheral region. That is, it was identified that, in the direct adding region, the crystal grows in a direction perpendicular to a substrate, whereas in the diffused region in the periphery thereof, the crystal grows in a direction horizontal to the substrate. It is assumed that they are caused by the differences in the initial nucleus generation of the crystal. That is, it is understood that, in the direct adding portion, these foreign matter comes into the crystal nucleus from which a crystal grows in the form of a column, whereas in the peripheral diffused region, the crystal nucleus is the direct adding portion growing in the longitudinal direction as mentioned above, and the lateral growth necessarily occurs because the growth starts from the crystal nucleus. Hereinafter, in this specification, the crystal growth region in the lateral direction extending from the direct adding region of the catalytic metal for low temperature crystallization to the periphery thereof is referred to as xe2x80x9clateral growthxe2x80x9d region.
Subsequently, the electric characteristics of the small-amount addition portion and the lateral growth portion in the vicinity thereof will be described in the case of using the above-mentioned metals, in this example, indium which belongs to the low melting point metal. As to the electric characteristics of the small-mount addition portion, the electrical conductivity is the same value as that of a film to which the metal has not been added, that is, a film obtained by crystallization at about 600xc2x0 C. for several ten hours. Also, as a result of obtaining the activation energy from the temperature dependency of the electrical conductivity, in the case where the amount of adding tin is 1017 atoms/cm3 to 1018 atoms/cm3 likewise as described above, no behavior caused by the level of indium (In) was observed. That is, it is considered from this experiment, that the crystalline silicon film can be used as the active layer of the TFT or the like if it has the above-mentioned density.
On the contrary, the electrical conductivity of the lateral growth portion is higher than that of the direct small-amount addition portion by one figure or more, which is a remarkably high value as a silicon semiconductor having crystallization. This is considered to be caused by the fact that a grain boundary existing in a space through which electrons pass between electrodes is reduced or hardly exist because the path direction of electric current coincides with the lateral growth direction of the crystal. This fact coincides with the result of a transmission electron beam microscopy photograph with no inconsistency. That is, it can be considered that since carriers moves along the grain boundary of the crystal which grew in the form of a needle or a column, the state where the carriers are liable to move is realized. The density of In in the region of the lateral growth was {fraction (1/10)} of that in the region where In was directly added. This is useful in further utilizing the crystalline silicon film without influence of In.
Now, finally, taking the above-mentioned various characteristics into consideration, a method of application of the metal to the TFT will be described. An active matrix type liquid crystal display unit using the TFT for driving pixels is supposed as the field of the TFT application.
As mentioned above, in the recent large-screen active matrix type liquid crystal display unit, it is important to restrain the contraction of a glass substrate. Using a process of adding a small amount of metal catalyst for crystallization at a low temperature, crystallization can be made at a satisfactorily low temperature in comparison with the strain point of glass, which is particularly preferable. According to the present invention, a portion which has been conventionally made of amorphous silicon is crystallized for about four hours at 500 to 550xc2x0 C. after adding a small amount of the low melting point metal or the catalytic metal whereby the portion can be readily replaced by crystalline silicon. Naturally, it is necessary to modify a design rule or the like correspondingly. However, it is considered that both of the conventional device and process can be sufficiently available, resulting in great advantages.
Furthermore, according to the invention, the TFT used for pixels and the TFT forming a driver for the peripheral circuit can be separately manufactured by utilizing the crystal form depending upon the respective characteristics, and there are many advantages more particularly in application of the invention to the active type liquid crystal display unit. The TFT used for the pixels does not require the mobility so much, and rather has a great advantage to reduce an OFF-state current.
For the above reason, a small amount of the low melting point metal or catalytic metal is added to a region which should come into the TFT used for the pixel so that a crystal grows in the lateral direction, as a result of which a large number of grain boundaries can be formed in the channel direction thereby to lower the OFF-state current. On the contrary, the TFT forming a driver for the peripheral circuit requires a very high mobility in view of the application of the TFT to a workstation, etc. Therefore, in the case of applying the present invention, a small amount of low melting metal or catalytic metal is added in the region close to the TFT forming the driver for the peripheral circuit, and a crystal is allowed to grow in one direction from the metal added portion. The direction of the crystal growth coincides with the path direction of electric current in the channel, thereby manufacturing the TFT having a very high mobility.
For reference, shown in FIG. 4 is an example where the Ni density after crystallization was investigated by SIMS in an example of obtaining a crystalline silicon film using Ni as the catalytic metal. It is apparent from FIG. 4 that the Ni density in the portion (lateral growth) where a crystal grew in a direction parallel to a substrate is lower than that in the region where Ni was directly added (plasma treated). Also, the data labeled with xe2x80x9ca-Sixe2x80x9d is the one to which no Ni is added, and the value is to be understood as a background. Similarly, in the present invention, it is considered that data having the fundamentally same tendency as that of FIG. 4 can be obtained. Therefore, it is considered that it is useful to utilize the region where the crystals grew in the direction parallel to the substrate.
If a silicon film crystallization is conducted by using a group III element, since the group III element remains in the film after the crystallization, a crystalline silicon film having a P-type conductivity can be obtained. Similarly, if the same is operated by using a group V element, a crystalline silicon film having n-type of conductive type can be obtained. The conductivity of the above crystalline silicon film with one conductive type can be controlled by the amount of group II and IV elements added when introduced at crystallization. Moreover, a conductivity type and a conductivity can be controlled by adding foreign matters that provide one conductivity type.
Further, for example, in the case where In which is a group III element is selectively introduced in the region 100 (FIG. 1A), and thereafter amorphous silicon film 104 is formed thereon so that crystallization is made by heating at 550xc2x0 C. for four hours, a crystal growth is made from the region 100 in a direction parallel to a substrate. At this time, since In is diffused as the crystal grows, the indium element exists in the region where the crystallization has occurred. Since the density is about 2xc3x971017 to 2xc3x971019 atoms/cm3, the region is crystallized and also made p-type. The p+region or pxe2x88x92region can be obtained by selecting the diffused region according to the amount of introducing In or crystallization. Then, TFT is formed by use of this region so whereby TFT having the channel forming region of p+type or pxe2x88x92type can be obtained. Likewise, in the case of using Sb which is a group V element instead of the above-mentioned In, the TFT having the channel forming region of n+type or nxe2x88x92type can be obtained. Thus, a threshold voltage of the TFT can be controlled by setting the conductive type of the channel forming region to pxe2x88x92type or the nxe2x88x92.
In the present invention,a trace amount of low melting point metal or catalytic metal as mentioned above is used for promoting a crystallization of a silicon semiconductor. The crystallization proceeds from the region where the metal has been added in one dimensional manner in a direction parallel with a substrate. The present invention is characterized by the use of such crystallized region for an electronic device.
More particularly, when an insulated gate field effect transistor is formed by use of a crystalline thin film silicon semiconductor of this region, a direction of moving carriers substantially coincide with a direction in which the crystals of a silicon film grow in its channel forming region, thereby being capable of obtaining a TFT having a high mobility. Further, the use of a crystalline silicon film whose crystals grew in a direction parallel to the substrate is useful in subjecting a diode and a transistor to integration for formation. Still further, a capacitor, a resistor and the like can be subjected to integration on the same substrate. Furthermore, they can be constituted by using an inexpensive glass substrate, which is another feature of the invention.
In the semiconductor using the thin film silicon semiconductor, the crystal growth direction of the crystalline silicon film whose crystals grew in the form of a needle or a column in the direction of a film plane substantially coincide with the direction of moving the carriers so that the carriers can move along the crystal grain boundary, thereby being capable of moving the carriers at a high mobility.